This invention relates to normally tacky and pressure-sensitive adhesive tape, and especially to tape suitable for use as a general purpose masking tape. The invention also relates to a method of making such a tape.
The use of pressure-sensitive adhesive tape to mask areas where it is desired not to apply lacquer, paint, etc. dates back well over half a century; cf., e.g., U.S. Pat. No. 1,760,820. This patent discloses the use of creped paper as the backing for the tape, enabling it to be stretched and shaped to conform to curved lines. Many improvements have subsequently been made in the original creped paper masking tape products, including calendering the creped paper in order to reduce its abrasive properties, saturating the paper with rubbery materials, etc. Creped paper masking tape can be severed with a knife, scissors, or a dispenser; it can even be torn by hand, although it is almost impossible to achieve a straight line tear, a fact that sometimes inconveniences and annoys users of the product.
U.S. Pat. No. 3,396,837 discusses conventional creped paper-backed masking tape, disclosing a replacement that is based on a particular kind of pigmented isotactic polypropylene resin film. This film-backed masking tape is said to have a much greater stretch value than typical creped paper masking tape, thereby permitting it to be conformed along severely curved lines while providing a sharp line of demarcation between the masked and unmasked areas. Tape made in accordance with this patent has been commercially available for many years, and, although it provides an inexpensive replacement for creped paper masking tape and is useful in a number of applications, it suffers from certain disadvantages. For example, it is difficult to initiate tear using only one's fingers, the Elmendorf tear test (ASTM Test D689-79) showing a value of 80-100 grams per ply. (By way of reference, the Elmendorf tear values for satured creped paper masking tape are in the range of 45-75 g/ply. Additionally, like conventional masking tape products, it is almost impossible to tear in a straight line. A third disadvantage is that, when subjected to a tensile force, the tape does not elongate evenly but instead stretches at what is apparently its weakest portion, resulting in a "necking down," or narrowing and thinning, of one portion of the backing while the remainder retains its original dimensions.
U.S. Pat. No. 4,139,669 discloses a "non-knifing plastic adhesive tape" which is said to be "hand tearable" by virtue of "striations," or grooves, that extend across the width of the tape. Depth, width, and pitch of the striations is dependent on the thickness and type of material of the film base used. The 14 working examples utilize polyvinyl chloride plasticized with dioctyl phthalate, an overall thickness of 0.105-0.275 mm, the depth of striation ranging from 0.01 to 0.09 mm and constituting 6.1 to 47.6% of the total thickness. Although the tabulated examples are somewhat hard to read, it appears that the transverse tear strength of the striated films ranges from 35 to 350 grams per ply. This product is not suggested for use as a masking tape and is not understood to be commercially available.
A somewhat related product is shown in U.S. Pat. No. 4,237,889, which describes embossing untensilized crystalline isotactic polypropylene or linear high density polyethylene film with a pattern of alternating ridges and valleys on one side, one possible conformation having the valleys extending laterally across the tape at right angles to the sides; cf. FIG. 1 of the patent. The specific use disclosed is in the preparation of tape closures for disposable diapers, where the ridges and valleys extend parallel to the long dimension of the closure, the ridges extending across the presumed line of tear and preventing any tendency of the closure to be torn inadvertently. This patent discloses the extrusion, embossing, and quick quenching of polypropylene film to obtain a product that is tough and ductile, unsuitable for a masking tape.